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Old 11-27-15, 08:36 AM   #1821
superlen
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More thoughts...

I think that any increase in heat transfer due to turbulent will only be from the water to the plastic pipe shell. From there is has to flow into earth which will be massively slower (an assumption & I know different locations are better/worse), so your overall loop field heat transfer is likely to not change any measurable amount. I think of it like a 1 ohm resistor in series with a 100k ohm resistor....you still got a big *** resistor. Resistor = R value in this analogy. I do need to check what an average R value is for dirt vs for plastic pipe.

Now if you are in a pump and dump, or pond, or any HX where the secondary medium is great heat conductor such as water vs the earth, then I think that there are some overall gains to be had.

Len

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Old 11-27-15, 09:38 AM   #1822
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Steve,

You keep saying that the flow in these loops is always waaay down in the laminar range, but my instinct tells me that's too far in the weeds. Also, when I do some research there are many posts where laminar vs turbulent pops up & is discussed at length. The GPM bandied about in those posts to get turbulent is no where as high as your numbers or suggestions are. Something seems amiss.

Are you confident with your calculations? I would hate to leave a post in this thread with misleading information.

Len
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Old 11-27-15, 02:46 PM   #1823
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Read the paper attached to my post and, as always, do your own calculations.

Steve
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Old 11-27-15, 02:58 PM   #1824
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Read the paper attached to my post and, as always, do your own calculations. I would welcome you doing this!


Steve
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Old 11-27-15, 03:24 PM   #1825
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Steve,

I understand the equation. I have done the calculations myself and you're answer is wrong. If you like, I can point out the error but I thought that you might want to discover it yourself.

It's a simply units mistake (I kept making it too trying to verify this) that makes you're answer 1000 times too low. The Reynolds number in my loops are approx 12000, not 12.8! That makes the flow highly turbulent, not highly laminar. Using the typical flow rates in most residential GEOs I would guess that almost every residential ground loop is flowing turbulent.

Disclaimer: I'm not a geo consultant, nor PE, nor do I claim to be anything more than a interested nerd in this forum, so I could well be totally wrong, but my numbers match the online calculator & also pass the sniff test when comparing to claims/calculations of others about turbulent loop flow in geo thermal fields. I'd be willing to bet a cold beer on them.

Len
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Old 11-27-15, 04:16 PM   #1826
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Len - I think I know where the error is - in the Pascals.sec number. That said, why would the energy to push fluid through your field be so low?

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Old 11-27-15, 05:14 PM   #1827
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Steve,

I too think it's the Viscosity you are using, all the other values in your equation looked correct to me. I think its' a Kg vs g discrepancy. For reference, here is the online calculator and equations I was using.

Absolute, Dynamic and Kinematic Viscosity

It yields exactly 1000 times greater Reynolds number than you calculated.

As for the low power requirement I calculated. I don't know, that's what prompted me to start digging into the equations. I felt that the pumping power that I calculated seemed low, hence the request for someone to chime in. (Thanks again, btw) I believe the pipe friction loss tables supplied by the pipe mnf already take into consideration laminar/turbulent. They are just head loss vs gpm irregardless of what's going on in the pipe, so ft of head * lbs of water moved during a particular time period should give ft-lbs/time.

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Old 11-28-15, 02:20 PM   #1828
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Quote:
Originally Posted by superlen View Post
More thoughts...

I think that any increase in heat transfer due to turbulent will only be from the water to the plastic pipe shell. From there is has to flow into earth which will be massively slower (an assumption & I know different locations are better/worse), so your overall loop field heat transfer is likely to not change any measurable amount. I think of it like a 1 ohm resistor in series with a 100k ohm resistor....you still got a big *** resistor. Resistor = R value in this analogy. I do need to check what an average R value is for dirt vs for plastic pipe.
I think your resistor analogy is mostly correct, but IGSHPA specifically recommends designing for turbulent flow, and they weren't just talking about water source heat pumps.

I think that there is an error in your application of the analogy of the serial resistors.

The temperature at the inner surface of the pipe wall, and the changes from different flows (laminar & turbulent) and the resulting 'delta-T' should not be viewed as a resistance in the string but as a voltage applied to the string.

This makes a big difference.

Best,

-AC
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Old 11-28-15, 03:00 PM   #1829
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AC,

After doing some more research, I am finding out that the thermal conductivity of the pipe vs the ground is actually not that different. My assumption was that the dirt was far less conductive, but that's not really the case. In most cases the plastic is *less* conductive than the dirt which matches what the IGSHPA general recommendations are....turbulent = good!

If you can get the heat from the water to the pipe, it will flow through the dirt as fast or faster. So any extra heat transfer you can get due to turbulence will be utilized easily. This would also explain why pretty much every residential installations have GPMs that are firmly in the turbulent range, NOT laminar. Note: One could get the same heat transfer with a much larger loop field & run in laminar mode, and this would result in a more efficient system....However, the installation cost of the larger field would most likely outweigh the small increase in efficiency over the life of the system. If you have your own backhoe and time, it *might* be worth it.

Len
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Old 11-30-15, 12:54 AM   #1830
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Quote:
Originally Posted by superlen View Post
If you can get the heat from the water to the pipe, it will flow through the dirt as fast or faster. So any extra heat transfer you can get due to turbulence will be utilized easily. This would also explain why pretty much every residential installations have GPMs that are firmly in the turbulent range, NOT laminar. Note: One could get the same heat transfer with a much larger loop field & run in laminar mode, and this would result in a more efficient system....However, the installation cost of the larger field would most likely outweigh the small increase in efficiency over the life of the system. If you have your own backhoe and time, it *might* be worth it.
Excellent research Len!

Well stated for others to follow.

Thank you.

-AC_Hacker

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